XA10 autofocus underwater - which one?

The Canon XA10 has three autofocus speeds: Instant, Medium and Normal.

Which one is most usable underwater?

As a side issue, the L&M XA10 housing's internal red filter holder blocks the secondary autofocus sensor located just below the lens. It blocks the sensor whether the filter is "in" or "out." If I remove the flip mechanism entirely along with the filter, I could possibly use the flat port and have the sensor function as designed, allowing me to use the faster focusing capability.

For the wide angle port, I probably can't use the two faster autofocus modes anyway because the port itself may block the sensor.

So is "Normal" autofocus OK? Is "Instant" more desirable? Or does it make no difference? Would any of these modes reduce "hunting" underwater with lower contrast subject matter?

There are 2 systems for the autofocus on the XA 10. One is TTL and the other (situated just under the lens) is IR. They work together, but if the IR is not working (depending on environnement conditions) the camera rely on the TTL only.
So you can forget the IR as you are underwater. I have a HFG10 in a L&M housing and I never care about which one (TTL or IR) is working as it is only the TTL which works UW.
Then you can forget the different mode and use only the Instant AF.
You will be able to use your flip red filter with no problems as the IR autofocus isn't working underwater.
These are the settings I use and it is all right.

It is worth noticing that if the auxiliary lens setting is selected from the camera's menu, the secondary sensor is turned off and "the AF mode is automatically set to Normal."

Therefore if the secondary sensor is blocked, the Normal autofocus is most likely the only one available because it uses TTL. The other two autofocus speeds require the secondary sensor to function in addition to TTL.

On a related note, the manual does state that "Autofocus takes longer to focus when the frame rate is set to PF30, PF24 or 24P than when it is set to 60i."